Here's an idea - check the VINs of those cars at carfax.com. The VIN is the 17-character string visible on the driver side windshield pillar. It'll be something like WDBCA39... for the SEL. There's a cost to checking so maybe someone on the forum with an open account can offer to help you. It won't tell you anything about the history of a car outside the US, but if carfax has any record of those cars then they were originally sold/registered in the US. A report might even validate the odometer readings. And what if they're reported as stolen in the US? I hope I'm not scaring you

Sixto
95 S420
91 300SE
87 300SDL
83 300SD